1. Field of the Invention
This invention relates to apparatus and methods for the preparation of substrates and deposition processes used in the compound semiconductor and related industries. More particularly, the invention relates to molecular beam epitaxy (MBE) apparatus and methods. The apparatus and methods of this invention provide an improved MBE growth chamber. 2.
2. Background Information
Molecular beam epitaxy is a growth process which involves the deposition of thin films onto a heated crystalline substrate in a vacuum by directing molecular or atomic beams onto the substrate. Deposited atoms and molecules migrate to energetically preferred lattice positions on the substrate, which is heated, yielding film growth of high crystalline quality, and optimum film thickness uniformity. MBE is widely used in compound semiconductor research and in the semiconductor device fabrication industry, for thin-film deposition of elemental semiconductors, metals and insulating layers.
Referring to FIG. 1, in MBE an effusion cell or source 100 emits gaseous molecules 102 in a beam 104 into an ultra high vacuum environment inside chamber 106. The beam 104 is directed at a predetermined angle toward a substrate (not shown) which is mounted in chamber 106. Each cell 100 deposits a selected element or molecule on the substrate.
Effusion cells exist in a variety of configurations with each configuration tailored for the specific material to be effused. Effusion cells 100 typically have a crucible 110 which contains the effusion material 112, for example aluminum, gallium, indium, or other compounds. The crucible 110 is resistively heated to effuse the material out of an orifice 114 into the chamber 106. The intensity of the beam is controlled by the temperature of the crucible. On each cell 100 a shutter 108 positioned near orifice 114 typically controls the flow of beam 102 by redirecting the beam away from the substrate.
High purity gases such as hydrogen and high vapor pressure materials such as arsenic, phosphorous, selenium and sulfur may also be effused by thermally cracking them into dimers or atomic species by means of a high temperature filament or other resistive heating element in an effusion cell. Rather than controlling the flow of the beam with a shutter, a valve can be used with gaseous materials to completely stop the flow of material from the cell.
A variety of crucible designs and effusion cell configurations are used in MBE. A discussion of advantages and disadvantages of various crucible designs is contained in U.S. patent application Ser. No. 08/433,033 entitled UNIBODY CRUCIBLE AND METHOD OF MANUFACTURE THEREFOR, and which is incorporated by reference herein.
In FIG. 2, a plurality of cells 100 are mounted, via ports 116, in a cooled growth chamber 106 which has an ultra high vacuum. Cells 100 can be actuated individually or in conjunction with other cells. Beams 104 are all directed at a substrate 118. The interior of chamber 106 is cooled by cryogenic material 120 circulating inside cryopanels 122, 124, 126, and 128 which are located inside chamber 106.
Regardless of which effusion cell design is used, problems exist with respect to the operation of the multiple cells. Because the effusion cells discharge into a central chamber, it is difficult to measure and control the precise amount of any one material in a mixture when multiple cells are activated simultaneously. It is undesirable to place a sensor directly in the path of the beam, which would create a shadow and disrupt the beam. An sensor placed near the substrate can detect the amount of the mixture, but not the percentage of individual components.
Another problem exists particularly in relation to the cryopanels, which are the typical chamber cooling means. The beams 104 are not actually so tightly focused as FIG. 2 appears to indicate. Molecules diffuse throughout the vacuum chamber depositing themselves on all the surfaces in the chamber. There is also "overspray" from the beams 104 which results in deposits on the horizontal portion 124 of the cryopanels adjacent to the substrate 118. Also, since a shutter merely redirects a beam 104 rather than stopping it, effusion material is redirected toward the vertical side 126 of the cryopanels. Much of the effusion material accumulated on the cryopanels is located above the effusion cells. Accumulated material can fall off the cryopanels onto an effusion cell, which could disrupt the flow of material from that cell, and could contaminate the cell if the material fell into an open crucible.
Despite the need in the art for a chamber design which overcomes the disadvantages, shortcomings and limitations of the prior art, none insofar as is known has been developed or proposed.
Accordingly, it is an object of the present invention to provide an improved chamber design. A specific object of this invention is to provide a growth chamber design which utilizes isolation chambers in a particular orientation with respect to the effusion cells. A further object of the present invention is to provide for measurement and control of the amount of individual components in a mixture of components effused from separate cells. A further object of the present invention is to shelter the effusion cells from falling debris accumulated on cryopanels.